The present invention relates to a carrier material and a desulfurization agent for use in reducing the sulfur content of molten iron.
The presence of sulfur, in most types of steel, above about 0.015% is undesirable. Sulfur is known to cause xe2x80x9chot-shortnessxe2x80x9d of steel during the hot rolling step of sheet steel processing. The presence of a sulfur containing liquid phase along grain boundaries of the metal causes a weakness of the metal at hot rolling temperatures.
Sulfur is most conveniently removed either 1) from molten iron, for example, after tapping from a blast furnace; or 2) from molten steel, for example, after processing in a BOP (basic oxygen process) vessel. Reducing sulfur content from iron following tapping from a blast furnace is preferred as the presence of carbon with the iron enhances the chemical reaction for removing the sulfur.
The practice of converting molten iron to steel as carried out in a BOP vessel by blowing oxygen into molten iron is known to reduce the sulfur content without additional treatment. That reduction in sulfur however is often not sufficient to meet the specifications of the grade of steel being produced and additional treatment must be carried out on the molten metal.
Various processes are described in U.S. Patents for lowering the sulfur content beyond that possible in a BOP vessel. They are carried out both on molten iron and molten steel.
U.S. Pat. No. 4,853,034 describes a process carried out on steel in a ladle wherein a calcium aluminate slag to which MgO is added is used to lower the sulfur content of the steel.
In U.S. Pat. No. 5,397,379 recycled LMF (ladle metallurgy furnace) slag is used for treating steel in a ladle. It is of a particle size between 1 inch and +20 mesh and is preferably added to the ladle while the steel is being tapped.
U.S. Pat. No. 5,972,072 describes a method of treating liquid iron wherein a desulfurization material is injected using a lance with a carrier gas or is carried out by adding the material as the hot metal is being poured into a ladle. The desulfurization material is made up of 3-20% particulate metallic aluminum, 5-30% alumina, 0.5-12% particulate hydrocarbon material, and the balance lime.
Many of such prior practice methods and materials result in a slag which is heavily laden with droplets of molten iron. Such condition results in a significant loss of iron with slag removal and a decrease in iron yield.
The present carrier material and desulfurization agent, and preferred process for their use reduces the sulfur content of liquid iron while the iron is contained in a hot metal vessel such as a transfer ladle and forms a slag which is easily skimmed from the molten iron surface and which entraps less iron than prior practice materials.
In a process wherein the carrier material of the invention is used, a lance is provided to direct the flow of a pressurized transport gas to which a particulate high percentage calcium aluminate carrier material is added. The flow of the transport gas and carrier material is initiated in the lance and the lance is then inserted into the molten iron. As the flow continues particulate magnesium (Mg) combined with up to 10% of lime (CaO), referred to herein as passivated magnesium, is added to the flow. The flow continues for a treatment period determined by the initial sulfur content of the iron and the final sulfur content desired. At the conclusion of such treatment period, the passivated magnesium flow is terminated with the gas-carrier material flow continued until the lance is raised above the slag surface. All of the particulate carrier material injected into the iron is of a particle size of about 200 mesh (85% minus 200 mesh and 100% minus 20 mesh). The carrier material is calcium aluminate combined with lime and preferably calcium fluoride and contains, by analysis, from about 54-74% CaO, from about 19-32% Al2O3, from about 0-4% MgO, and from about 0-10% CaF2 (all percentages being weight %). Impurities often associated with calcium aluminate, which can be detrimental to the desulfurization process, are kept below the following levels: SiO2-2.5% max, Fe2O3-1.0% max, P2O5-0.025% max and TiO2-0.025% max MnO-0.5% max, V2O5-0.025 max, K2O-0.025 max, and S-0.05 max. A combined LOI(loss on ignition)/moisture level of the material is less than 1.5%.
In a process wherein the desulfurization agent of the invention is used, a pre-blended mixture of the above carrier material and selected amounts of passivated magnesium material are delivered in the combined state from a pressurized vessel to the lance for injection.
In a third embodiment of the invention a narrow compositional range of the carrier material is used both before and following known desulfurization treatments to improve slag characteristics and increase iron yield.
Other specific features of the invention are described in more detail in relation to the accompanying drawings.